Fetal pericardial effusion after maternal COVID-19 vaccination: a fortuitous association?

Introduction: Studies monitoring COVID-19 and its vaccines are warranted to expand knowledge about the virus and its effects in humans.The objective of this study was to compare the reasons for referral and the fetal echocardiographic results during (PAND: Mar/20-Aug/21) and before (PREP: Set/19-Feb/20) the COVID-19 pandemic.Material and methods: Single outpatient centre, retrospective study.Results: A total of 10,778 fetal echocardiograms were performed; 10,732 corresponding to initial examinations performed in 10,551 pregnancies (10,376 singletons; 171 twins; 2 triplets; 2 quadruplets).Only first-time exams were reviewed.There were 5652 during PREP and 5080 during PAND.Maternal ages (X) varied between 14 and 46 years (X -PREP = 29.1andX -PAND = 29.2),and gestational weeks (Y) varied between 16 and 39 weeks (Y -PREP = 26.9 and Y -PAND = 27.6).The exams were not performed in women with known active COVID-19 infection.In PREP, more women were referred due to maternal or fetal risk factors.During PAND, more women were referred due to a suspected heart defect on ultrasound.During PAND we identified more fetuses with congenital heart disease and arrhythmias.A higher incidence of pericardial effusion, without structural heart disease, was observed during the months of June, July, and August 2021, compared to the remaining months (p < 0.001).Conclusions: The identification of more structural heart disease during PAND may reflect changes in referral patterns.The increase in isolated pericardial effusion coincided with the period of massive COVID-19 immunization.We hypothesize that this finding may reflect a transient inflammatory response that could signal to fetal immunization against SARS-CoV-2.